| dc.description.abstract | Abstract :Cardiac hypertrophy has been identified as the most important independent risk factor
for cardiovascular-related morbidity and mortality and is therefore regarded as
a pathological condition. Despite this, beneficial physiological forms also appear to
exist, such as in response to exercise, leading to maintained or improved cardiac
function. The aim of this thesis was to examine two distinct rodent models, an endurance
run-trained rat, and the DOCA-salt hypertensive rat, representing physiological
and pathological hypertrophy, respectively, in order to develop a better understanding
of the molecular changes associated with each condition. The thesis also examined
the effect of dietary supplementation of L-arginine to the pathological model,
a treatment that has been shown to ameliorate/prevent many of the cardiovascular
impairments. Studies examined selected candidate genes (qRT-PCR), including conventional
biomarkers of hypertrophy and exploratory analysis of adenosine-related
genes (given adenosine’s established regulatory and protective role in the heart, yet
minimally studied in cardiac hypertrophy), and explored global transcriptomic shifts
via microarrays. The hypothesis of this work was that cardiac hypertrophy lies on
a continuum, with similarities existing at the cardiac transcriptional level between
early (adaptive) stages of pathological hypertrophy (DOCA-salt rat) and later stages
of physiological hypertrophy (endurance run-trained rat).
Examination of ten biomarkers of hypertrophy (ANF, BNP, -MHC, -MHC, cardiac
-actin, skeletal -actin, SERCA2, PPAR, Coll I and III) revealed that the pathological
model displayed alterations in the expression of many of these molecules in
line with the literature. These changes were not observed in the physiological model.
This therefore reinforces the value of conventional biomarkers in delineating pathological
vs. physiological hypertrophies, and reveals fundamental differences in genesis
of these two forms of hypertrophy.
The adenosine system (receptors and purine handling molecules) was altered in
the pathological hypertrophy model as evidenced by the modulation of genes corresponding
to A3AR, Ada, and Adk, with a potential shift from purine salvage towards
degradation of adenosine to inosine. Furthermore, this study represents the
first report of altered regulation of the nucleoside transporter ENT3 in a pathological
condition. None of these changes were seen in the physiological model with only
modulation of the A2aAR evident.
Examination of the transcriptional response to physiological hypertrophy revealed
that short (6 week) and long (12 week) training programmes resulted in different profiles,
likely reflecting progression of the hypertrophy process. The short programme
stimulated genes associated with the mitochondria, oxidoreductase, receptor binding
and coenzymemetabolismand repressed the expression of transcripts associated with
phosphorylation, catalytic activity, defence/immunity and energy pathways. Thus,
initial changes observed are primarily of a metabolic and signalling nature. In contrast,
the longer programme resulted in shifts in protein handling and synthesis, and
genes involved in structural molecule activity, nucleotide binding and cellular homeostasis.
These patterns support a progression with time from initial metabolic adaptations
to longer term shifts in protein phenotype and structural adaptations, consistent
with longer term changes in heart structure.
Similarly, the pathologicalmodel displayed different time-dependent gene expression
profiles. Overall, the pattern of changewith early (2week) treatment is suggestive
of changes in intracellular signalling and increasing transcriptional capacity with the
later changes (at 4 weeks) indicative of structural adaptations (intra- and extracellularly)
togetherwith an inflammation response. Genes coding for calciumhandling, ion
channels, and gap junctions were altered throughout themodel andmay contribute to
electrical conduction defects and cardiac dysfunction. The adrenergic signalling pathway
was modulated as associated signalling molecules were down-regulated. The
study revealed many expected and novel changes, of which further study should focus
on: calcium regulation, metabolic regulation, gap junctions, and (as might be exii
pected) signalling via the adrenergic pathway, insulin-like growth factor, PI3K, and
Jak/STAT.
L-Arginine modulated biomarker expression in pathological hypertrophy, with
stimulation of PPAR and SERCA2 with little or no effect on the adenosine-related
genes. L-Arginine affected the overall transcriptional response to DOCA-salt treatment,
stimulating genes involved in cell growth andmaintenance, nerve transmission,
heparin and glycosaminoglycan binding, peptide binding and protein targeting, as
well as the repression of genes related to apoptosis (favouring a pro-apoptotic state),
intracellular organisation and biogenesis, and enzyme inhibitor activity. The beneficial
effects of L-arginine in the setting of pathological hypertrophy may be due to
modulation of metabolism, improving calcium handling and overall enhancing cellular
functioning.
This work demonstrates that cardiac hypertrophy is clearly different at the transcriptional
level depending upon the aetiology. This repudiated the hypothesis of the
thesis that cardiac hypertrophy lies on a continuum with similarities existing at the
cardiac transcriptional level between early (adaptive) stages of pathological hypertrophy
and later stages of physiological hypertrophy. Whilst some of the data was in accordance
with current knowledge of these states, novel changes were also discovered,
contributing to our understanding of the molecular aspects of cardiac hypertrophy. | |
